CHICAGO (June 26, 11 a.m. EDT) — At North America's largest plastics trade show, a key rival material, wood, attracted a lot of attention.
Several manufacturers of extrusion systems highlighted new wood-fiber/plastics composite systems. And the reputed largest supplier of wood fiber to the plastics industry reported massive traffic at its booth.
Wood/plastics composites production is growing rapidly as the construction trade exploits the combined benefits of the two materials. The conservative construction industry feels most comfortable working with wood, notwithstanding its gradual acceptance of vinyl siding, window profiles, decking and fencing.
Adding plastics to wood makes it resistant to rot, insects and warping, making it much easier to convince builders that the combination can be ideal in many applications.
Moisture content in wood fiber and flour is what most extrusion systems suppliers are spending a lot of effort addressing. Moisture content can be as high as 12 percent, which presents a problem in extrusion systems that liberate the moisture as high-pressure steam.
Davis-Standard Corp. addresses the moisture problem upstream of the wood/plastic mixing process. Its Woodtruder removes moisture in the first section of the primary extruder with a unique heating and venting system.
A crammer feeder conveys the wood fiber into the primary extruder. Midway along the primary extruder, a side-injection extruder separately heats and mixes the plastic and injects it at a controlled rate into the primary extruder. Since the plastic is already in a molten state, it encapsulates the wood fibers to give a thoroughly mixed, high-quality composite that can contain as much as 80 percent wood fiber, according to the firm's business area manager of building products Dave Murdock.
Davis-Standard says the two-extruder approach allows processing of a variety of fibrous materials such as peanut shells, wood shavings and rice husks, and recycled materials such as crumb rubber. The Woodtruder's flexibility is being embraced by the market, according to a spokeswoman who said Davis-Standard sold eight of the machines in the week prior to NPE in Chicago.
Krupp Werner & Pfleiderer Corp. reports a lot of success for its wood-fiber extrusion systems. Its twin-screw compounder system uses a single-screw pump to feed wood fiber into the compounding extruder. Moisture is removed by mechanical energy and the steam escapes through strategically located atmospheric vents along the process section. The primary polymer and additives are compounded with the dried wood fiber in the ZSK co-rotating twin-screw machine.
Michael Kenny, Krupp W&P vice president and general manager, said his firm has several installations in the field, and the market represents "great potential for growth." Krupp W&P works with various downstream equipment suppliers for sizing calibrators, haul-offs and cutters to ensure product quality for market acceptance.
Berstorff Corp. is another extrusion systems supplier that has invested a lot in wood-fiber/plastic processing. Martin Mack, vice president of research and development for the Florence, Ky., firm, said his company's system uses a low-shear mixing element that wets out the fiber and steam is vented out in a special section.
"Steam velocity can cause problems in venting," Mack explained. "Our vent stuffer prevents steam from mixing with solids."
Berstorff's association with other Kraus-Maffei companies allows it to integrate downstream equipment with its extruders, Mack added.
Mack said wood fiber's sensitivity to temperature is a key consideration in extrusion systems.
In systems where wood fiber is pre-dried before extrusion, ideal drying temperatures are between 212° F and 300° F, according to Don Murray, sales manager of the industrial product division of American Wood Fibers Inc. of Schofield, Wis. Rotary dryers, fluid beds and similar dryers that expose a large surface area to heat typically are used by composite producers that don't remove moisture during the extrusion process, Murray said in an interview at NPE.
Murray claimed American Wood is the largest supplier of industrial wood fiber in the United States. This month it is opening its sixth wood-fiber facility, in Circleville, Ohio. Wood fiber has long been used as a filler in plastics, particularly in automotive interior trim, but structural composites are rapidly expanding usage of the fibers in plastics.
American Wood buys wood fiber and flour mainly from wood processors that generate the material in sanding, planning and other operations. It does not buy directly from sawmills because moisture content of that material can be as high as 20 percent.
Privately held American Wood mainly deals in oak, maple, Ponderosa Pine and Southern Yellow Pine fiber. The family-owned company claims to be the only wood-fiber supplier that is ISO 9002-certified. Its Schofield plant has the accreditation and is a model for quality systems at its other five production plants.
Mack said special waxes and metallic stearates are common additives to make wood fiber and plastics compatible.
He said hundreds of companies are in the wood/plastic composite business but he wouldn't disclose a precise figure "because we supply most of them." Players can run as few as one or two lines or can be much larger producers, such as Louisiana Pacific.
Mack said the next major step for wood/plastic composites are for them to achieve widely agreed-upon standards for properties. The National Window and Door Association has set tentative specifications and other associations are sure to follow, he predicted.
Berstorff's Mack said wood/plastic composite markets are far more advanced in North America than in Europe, particularly because of different building practices. Building a wood deck onto a house, for example, would be considered unusual in Europe, according to Mack. He does, however, expect markets eventually will develop on the other side of the Atlantic.
